Integrated receiver and ADC for capacitive touch sensing apparatus and methods

ABSTRACT

An integrated analog data receiver for a capacitive touch screen. An analog data receiver circuit for a touch screen device is provided including a sigma delta analog to digital converter configured for direct connection to an analog output of a touch screen device, and further including an integrator circuit having an input coupled for receiving the analog output signal and outputting an integrated output voltage; a comparator coupled to the integrated output voltage and a first bias voltage and outputting a comparison voltage; a clocked sampling latch coupled to the comparison voltage and to a clock signal and outputting quantized data bits corresponding to samples of the comparison voltage; and a digital filter and decimator coupled to the clocked sampling latch and outputting serial data bits which form a digital representation corresponding to the output of the touch screen device. Additional circuits and systems are disclosed.

RELATED APPLICATIONS

This patent application claims priority to U.S. Provisional ApplicationSer. No. 61/901,807, entitled “Integrated Receiver ADC Architecture forCapacitive Touch Screen Sensing,” filed Nov. 8, 2013, which is herebyincorporated by reference herein in its entirety.

TECHNICAL FIELD

Aspects of the present application relate generally to the use ofcapacitive touch sensing for touch screens as input devices.Applications of capacitive touch screen devices for input includeportable devices such as cell phones, smartphones, tablet computers,laptop computers, and the like, as well as personal computers.Additional applications of touch screens include input interfaces toconsumer devices and industrial devices having touch screen displays andinput panels.

BACKGROUND

In a system that uses a touch screen that combines user input anddisplay functions, projective capacitive touch screens are increasinglyused. These touch screens do not require pressure or even actual contactto detect a finger used to indicate an input at a location on a twodimensional touch screen. The use of projective capacitive touch screenson smartphones and tablet devices has increased markedly in recentyears. These touch screen devices can also be used in many otherapplications.

In a projective capacitive touch screen display, in one known approach,a grid of conductors are formed with rectangular conductors spaced fromone another by a dielectric. A capacitance is formed at each gridintersection by the presence of two plates (the conductor material)spaced by a dielectric. In a touch screen display, the conductivenetwork overlies a display device such as an LED/LCD display, and iconsor symbols may be displayed to indicate different actions that can betaken in response to a selection by a user, for example. The touchscreen therefore needs to be transparent to allow the user to view theunderlying display. Indium Tin Oxide (ITO) touch screens are often usedbecause this conductive material provides the needed transparency. Otherconductive materials may be used. A protective layer such as a glass orsimilarly hard and transparent material is provided over the touchscreen. When a conductive second element, such as a human finger,approaches the touch screen the capacitance in the area where the touchis occurring changes (a finger or conductive stylus which creates aparallel capacitive element proximate to the node), and this change canbe detected by circuitry coupled to the touch screen to indicate a touchat a specific location. Using this information, a processor can thendetermine what action is to be taken. For a few non-limiting examples,in a smartphone or tablet device, the processor can launch anapplication, start or stop a process, input a number that has beentouched, or change the display to another screen to reflect the user'sinput.

FIG. 1 depicts in a simple block diagram a device 10 such as asmartphone or tablet computer presented to illustrate an application fora projective capacitive touch screen. In device 10, a body or chassis 11supports a touch screen display 13. The device 10 is not limited tosmartphones and tablet computers; device 10 can be any device where auser interface using touch can be utilized. Examples include, withoutlimiting the scope of the present application, industrial controls,consumer controls such as thermostats, security alarms, door openers,home lighting, sound systems, video systems, medical devices such asmonitors and testers, portable devices such as pagers, music players,video players, fitness monitors, timekeeping devices, radios, stereos,televisions, set top boxes, laptop and “convertible” personal computers,desktop computers, workstations, and the like.

FIG. 2 illustrates a portion of a touch screen device 100 in operation.In FIG. 2, finger 110 is shown touching the surface 112 of a touchscreen device 100. The protective screen 108, which may be glass,sapphire glass, or another transparent layer such as polycarbonate thatprovides mechanical and moisture protection, is touched. Conductor 102forms a first plate of the capacitor Cp, the panel capacitor at thatlocation. Conductor 104 provides a second plate. The insulating layer106 spaces the two conductor layers and forms the dielectric for thecapacitor Cp. As can be seen in FIG. 2, as the finger 110 (which canalso be, for example, a conductive stylus or similar tool) makes contactor almost makes contact with the surface 112, the capacitor Cf is placedin parallel with the capacitor Cp and this changes the capacitance atthe location where the touch occurs. A touch screen panel can have tens,hundreds or thousands of the capacitors Cp in an array across the panel.The array of nodes is used to provide an accurate location for thetouch, so that the correct location is determined for use by the system.

FIG. 3 depicts a prior known solution for receiving the output of atouch screen panel and for providing a digital data output DO for use bya system 300 that includes the touch screen panel 301.

In FIG. 3, a portion of a touch screen panel 301 that senses thecapacitance change on sense capacitor C0 has an output labeled ‘e’ thatis coupled to an analog data receiver 310. The analog to data receivercircuit 310 can be implemented as one or more monolithic integratedcircuits. In a known prior solution, the circuit 310 is coupled to thepanel 301 by a ribbon cable or flex cable connection (not shown) whichadds resistance to the system 300. If monolithic integrated circuits arenot used, off-the-shelf and/or discrete components can be used insteadon a board such as a printed circuit board (PCB) or module to formcircuit 310.

In the analog data receiver circuit 310, the first stage 303 implementsa trans-impedance amplifier (TIA) and the second stage 305 implements,in this illustrative and non-limiting example, a Sallen-Key band passfilter. The output Vout is an analog voltage that is then converted byan analog to digital converter 311 (ADC) to a digital representation ofthe input signal ‘e’. The digital data outputs DO are then available forfurther processing by the system.

In operation, the sense capacitor C0 receives as an input a time varyingvoltage Vin such as a sinusoidal stimulus signal. Because the currentsourced from a capacitor is equal to the capacitance multiplied by thechange in voltage, e.g., dQ/dt=I=C dV/dt, the output ‘e’ of the sensecapacitor C0 from the panel 301 is taken as a current. Thetrans-impedance amplifier TIA 303 receives the analog current signal andoutputs a corresponding voltage signal to the band pass filter 305, andthe band pass filter circuit 305 then outputs the voltage Vout to theADC 311.

The architecture of the analog data receiver 310 (sometimes referred toas an “analog front end” or AFE) has several disadvantages. Toillustrate these, some further analysis of the circuitry is needed.

To obtain the maximum signal to noise ratio (SNR) possible, the inputvoltage Vin may be driven at a full voltage scale. In order to maintainthe same voltage domain at the receiver end, the capacitor Cf1 incircuit 303, the TIA circuit, must be less than or equal to the sensecapacitor C0. This arrangement attenuates the input signal, as shown inEquation 1:

$\begin{matrix}{1 = {\frac{e}{Vin} = {\frac{C\; 0}{{Cf}\; 1} < 1}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

The noise factor N.F. of the signal passed into the Sallen-Key filter305 can be written in terms of the Friis equation for multiple stageamplifiers, as shown in Equation 2:

$\begin{matrix}{{N.F.} = {{F\; 1} + \frac{{F\; 2} - 1}{G\; 1}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$Where F1 is the noise factor of the TIA stage 303, G1 is the gain of theTIA stage 303, and F2 is the noise factor of the second stage 305

The band pass filter nature of the entire system can be expressed as:

$\begin{matrix}{\frac{Vout}{{Vin}\left( {{Tx}\lbrack n\rbrack} \right)} = {\frac{{sC}\; 0{Rf}\; 1}{\left( {{{SCfRf}\; 1} + 1} \right)} \star \left( \frac{\frac{Ks}{C\; 1r\; 1}}{\begin{matrix}{s^{2} + {s\left( {{\left( \frac{1}{C\; 1} \right)\left( {\left( \frac{1}{R\; 1} \right) + \left( \frac{1}{R\; 2} \right) + {\left( \frac{1}{R\; 3} \right)\left( {1 - K} \right)}} \right)} +} \right.}} \\{\left. \left( \frac{1}{C\; 2R\; 2} \right) \right) + \left( \frac{{R\; 1} + {R\; 3}}{C\; 1C\; 2R\; 1R\; 2R\; 3} \right)}\end{matrix}} \right)}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

The angular center frequency ω0 that corresponds to the bandwidth beingpassed by the system is given by Equation 4:

$\begin{matrix}{{\omega\; 0} = \sqrt{\frac{\left( {{R\; 1} + {R\; 3}} \right)}{C\; 1C\; 2R\; 3R\; 1R\; 2}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

The bandwidth for the system is given by Equation 5:

$\begin{matrix}{{B.W.} = {{\left( \frac{1}{C\; 1} \right)\left( {\left( \frac{1}{R\; 1} \right) + \left( \frac{1}{R\; 2} \right) + {\left( \frac{1}{R\; 3} \right)\left( {1 - K} \right)}} \right)} + \left( \frac{1}{C\; 2R\; 2} \right)}} & {{Equation}\mspace{14mu} 5}\end{matrix}$

The Gain for the system is given by Equation 6:

$\begin{matrix}{{Gain} = \frac{{KC}\; 0}{C\; 1R\; 1{B.W.{Cf}}}} & {{Equation}\mspace{14mu} 6}\end{matrix}$

In considering the performance of the overall system, it can be seenfrom examining Equations 3 and 5 above that the bandwidth B.W. and thepeak frequency ω0 depend on the values of discrete components C1, C2,R2, R1, and R3, whereas the gain depends on K, C0 and inversely dependson C1, R1, B.W. and Cf, as given by Equation 6.

In order to increase the gain of the system, K (the gain of the op amp309) can be increased as seen in Equation 6. However, if K is increasedsignificantly, the band pass filter may enter an instability region, forexample it may begin to oscillate. Therefore K should be limited inorder to avoid instability or oscillation. The total harmonic distortion(THD) will also increase as K increases. This limits the ability of thecircuit designer to increase the gain.

The analog data receiver topology of the known prior solution analogdata receiver 310 that is illustrated in FIG. 3 also requires a highnumber of passive components, including 6 resistors, 3 capacitors, and 2active analog amplifiers. Then, to finally obtain a digital output, theprior known solution also requires an additional analog to digitalconverter (ADC) following the analog amplifiers. This circuit topologymakes integration of the analog data receiver 310 in a monolithicintegrated circuit difficult. Depending on the semiconductor technologynode used to manufacture the integrated circuits, multiple integratedcircuits can be required to implement the amplifiers and passivecomponents and to then form the analog to digital converter 311 tocomplete the analog receiver solution for the touch screen. Further, itmay not be possible to form multiple signal channels on a singleintegrated circuit using these prior known approaches.

The use of the large number of passive components on such an integratedcircuit makes process tolerances in semiconductor manufacturingdifficult, as each of the passive components has to be formed with aprecise value. As is known to those skilled in the art, when a circuithas a low tolerance to process and temperature dependent devicevariations, semiconductor manufacturing yields are lowered and coststherefore increase for the good devices that are produced. Even afterthe analog receiver data 310 is provided, additional components thatprocess the digital signals are still needed to complete the overallsolution, requiring still further circuitry. The silicon area needed toimplement the many passive components in analog data receiver 310 makesfurther integration with the ADC 311 or with other functions difficult,increasing board area and increasing the total number of integratedcircuits needed to complete a system with a touch screen display.

Improvements are therefore needed in the analog data receiver circuitryfor touch screen devices, such as for the analog front end of touchscreen systems, in order to address the deficiencies and thedisadvantages of the prior known approaches. Solutions are needed thatreduce the number of passive components, reduce the active analogcircuitry, increase tolerances to process and temperature devicevariation, and which improve the performance and increase the level ofintegration of the circuits.

SUMMARY

Various aspects of the present application provide novel analog datareceiver circuitry for capacitive touch screen devices with novelcircuit topologies that integrate the ADC and analog data receivercircuit functions together.

In one aspect of the present application, an analog data receivercircuit for a touch screen device includes a sigma delta analog todigital converter configured for direct connection to an analog outputsignal of a touch screen device, and further includes: an integratorcircuit having an input coupled for receiving the analog output signalfrom the touch screen device and outputting an integrated outputvoltage; a comparator coupled to the integrated output voltage and afirst bias voltage and outputting a comparison voltage; a clockedsampling latch coupled to the comparison voltage and to a clock signaland outputting quantized data bits corresponding to samples of thecomparison voltage; and a digital filter and decimator coupled to theoutput of the clocked sampling latch and outputting serial data bitswhich form a digital representation corresponding to the analog outputsignal of the touch screen device.

In another aspect of the present application, the analog data receivercircuit is provided wherein the integrator circuit further includes: anoperational amplifier having a positive input terminal coupled to a biasvoltage, and a negative input terminal coupled to the analog outputsignal and outputting the integrated voltage output; a capacitor coupledin a feedback path from the integrated voltage output to the negativeinput terminal; and a resistor coupled in parallel and in the feedbackpath from the integrated voltage output to the negative input terminal.

In yet another aspect of the present application, the analog datareceiver circuit described above further includes a summing node at theinput to the integrator circuit and coupled to the analog output signal;and a feedback path coupling an inverted analog feedback voltage to thesumming node.

In yet another aspect of the present application, the analog datareceiver circuit described above further includes a buffer in thefeedback path coupled to receive the output of the clocked samplinglatch and outputting the analog feedback voltage to the summing node.

In still another aspect of the present application, the analog datareceiver circuit disclosed above further including: a resistor coupledin the feedback path and disposed between the buffer and the summingnode. In a further aspect of the present application, in the analog datareceiver circuit described above, the buffer performs a digital toanalog conversion, and the analog feedback voltage has a levelcorresponding to a voltage represented by the serial data bits output bythe clocked sampling latch.

In yet another aspect of the present application, in the analog datareceiver circuit disclosed above, the comparator further includes: anoperational amplifier having a positive input terminal coupled to theoutput of the integrator circuit; and a negative input terminal of theoperational amplifier coupled to a bias voltage.

In still another aspect of the present application, in the analog datareceiver circuit described above, the clocked sampling latch furthercomprises an edge triggered data flip flop.

In yet another additional aspect of the present application, the analogdata receiver disclosed above is provided, wherein the decimator takessamples of the quantized data bits output by the clocked sampling latchand outputs a number of digital data bits lower than the number ofquantized data bits.

In yet another alternative aspect of the present application, the analogdata receiver described above is provided wherein the filter performs adigital band pass filtering of the serial data output by the clockedsampling latch.

In still another aspect of the present application, an integrated analogdata receiver circuit for a touch screen device includes: a plurality ofinputs configured to receive analog sense outputs of a touch screendevice; a plurality of analog data receiver circuits, each of theplurality of analog data receiver circuits coupled to at least one ofthe plurality of inputs, and each further includes a sigma delta analogto digital converter further comprising: an integrator circuit having aninput directly receiving an analog sense output from at least one of theplurality of inputs and outputting an integrated output voltage; acomparator coupled to the integrated output voltage and coupled to afirst bias voltage and outputting a comparison voltage; a clockedsampling latch coupled to the comparison voltage and to a clock signaland outputting quantized data bits corresponding to samples of thecomparison voltage; and a digital filter and decimator coupled to theoutput of the clocked sampling latch and outputting serial data bitsforming a digital representation corresponding to one of the analogsense outputs.

In another alternative aspect of the present application, in theintegrated analog data receiver circuit described above, within each ofthe sigma delta analog to digital converters, the integrator circuitfurther comprises an operational amplifier having a positive inputterminal coupled to a bias voltage, and a negative input terminalcoupled to the analog sense output signal and outputting the integratedvoltage output; a capacitor coupled in a feedback path from theintegrated voltage output to the negative input terminal; and a resistorcoupled in parallel with the capacitor in the feedback path from theintegrated voltage output to the negative input terminal.

In yet another aspect of the present application, in the integratedanalog data receiver circuit described above, wherein: within each ofthe sigma delta analog to digital converters, each of the comparatorsfurther comprise an operational amplifier having a positive inputterminal coupled to the integrated voltage output of a correspondingintegrator circuit; and a negative input terminal of the operationalamplifier coupled to a bias voltage.

In still another additional aspect of the present application, in theintegrated analog data receiver circuit described above, wherein: withineach of the sigma delta analog to digital converters, each of thedecimators sample the quantized data bits output by a correspondingclocked sampling latch and each decimator subsequently outputs a numberof digital data bits that is lower than the number of quantized databits sampled.

In still another additional aspect of the present application, in theintegrated analog data receiver circuit described above, wherein: withineach of the sigma delta analog to digital converters, the clockedsampling latch further comprises an edge triggered data flip flop.

In yet another aspect of the present application, in the integratedanalog data receiver circuit described above, wherein each of the sigmadelta analog to digital converters further comprises: a summing node atthe input to the integrator circuit; and a feedback path coupling aninverted analog feedback voltage to the summing node.

In another aspect of the present application, a system for receivinguser inputs from a touch screen device is provided that includes: atouch screen device configured to transmit analog output signalscorresponding to touches detected on a capacitive touch sensitivesurface of the touch screen device; a touch screen controller circuitthat is integral with the touch screen device, including a plurality ofanalog data receiver circuits, each of the plurality of analog datareceiver circuits coupled to at least one of the analog output signals,and each further comprising: a sigma delta analog to digital converterfurther comprising: an integrator circuit having an input coupled forreceiving an analog signal directly from at least one of the analogoutput signals and outputting an integrated output voltage; a comparatorcoupled to the integrated output voltage and coupled to a first biasvoltage and outputting a comparison voltage; a clocked sampling latchcoupled to the comparison voltage and to a clock signal and outputtingquantized data bits corresponding to samples of the comparison voltage;and a digital filter and decimator coupled to the output of the clockedsampling latch and outputting serial data bits forming a digitalrepresentation corresponding to an analog signal at the input; and adigital processing circuit coupled to receive the serial data bits asinput data and configured to perform defined functions responsive to thetouches detected on the touch screen device.

In yet another aspect of the present application, in the systemdescribed above, wherein the touch screen controller circuit comprises amonolithic integrated circuit including at least the plurality of analogdata receiver circuits.

In still another aspect of the present application, the system disclosedabove is provided, wherein the touch screen controller circuit comprisesa single monolithic integrated circuit including at least the pluralityof analog data receiver circuits and the digital processing circuit.

In yet another additional aspect of the present application, the systemabove is provided, wherein the touch screen device further comprises adevice that is one selected from the group consisting essentially of asmartphone, a tablet computer, a music player, a video player, a laptopcomputer, a desktop computer, a set top box, a consumer appliance, andan industrial control.

Recognition is made in aspects of this application of a solution for ahighly integrated analog data receiver circuit for a touch screen deviceby forming a circuit topology where a sigma delta analog to digitalconverter is directly connected to the analog output signals of thetouch screen device. The novel analog data receiver circuits discloseduse fewer components and eliminate many passive components that werepreviously used in the prior known solutions, enabling higher levels ofintegration and more reliable circuit operation and performance.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the illustrative examples ofaspects of the present application that are described herein and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates in a simplified a touch screen device illustrated fordescribing various aspects of the present application;

FIG. 2 illustrates in a cross sectional view a portion of a touch screendevice in operation;

FIG. 3 illustrates in a simplified circuit diagram an analog datareceiver circuit of the prior art;

FIG. 4 illustrates in a simplified block diagram a conventional sigmadelta analog to digital converter circuit;

FIG. 5 illustrates in a simplified circuit diagram an exampleillustrative arrangement of an analog data receiver circuitincorporating features of the present application;

FIG. 6 illustrates in a graph a comparison of the signal to noise ratioof a prior art analog data receiver circuit to the signal to noise ratioobtained using an analog data receiver circuit arrangement incorporatingaspects of the present application;

FIG. 7 illustrates in a simplified circuit diagram an alternativearrangement analog data receiver circuit incorporating aspects of thepresent application and implemented using a second order analog todigital converter;

FIG. 8A illustrates in a graph the signal to noise ratio obtained for anexample arrangement analog data receiver that incorporates aspects ofthe present application, and FIG. 8B illustrates in a graph the signalto noise ratio obtained for another example analog data receiverincorporating aspects of the present application;

FIG. 9 depicts in a simplified block diagram an integrated circuitincluding an arrangement including a plurality of analog data receivercircuits each incorporating features of the present application; and

FIG. 10 depicts in a simplified block diagram a system for a touchscreen and a touch screen controller integrated circuit incorporatinganalog data receiver circuits that include various aspects of thepresent application.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the illustrativeexample arrangements and are not necessarily drawn to scale.

DETAILED DESCRIPTION

The making and using of example illustrative arrangements thatincorporate aspects of the present application are discussed in detailbelow. It should be appreciated, however, that the illustrative examplesdisclosed provide many applicable inventive concepts that can beembodied in a wide variety of specific contexts. The specific examplesand arrangements discussed are merely illustrative of specific ways tomake and use the various arrangements, and the examples described do notlimit the scope of the specification, or the scope of the appendedclaims.

For example, when the term “coupled” is used herein to describe therelationships between elements, the term as used in the specificationand the appended claims is to be interpreted broadly, and is not to belimited to “connected” or “directly connected” but instead the term“coupled” may include connections made with intervening elements, andadditional elements and various connections may be used between anyelements that are “coupled.”

In various aspects of the present application, novel solutions areprovided to receive the analog output signals from a capacitive touchscreen device and to directly perform an analog to digital conversion toobtain digital data corresponding to the analog output signals. In anillustrative arrangement, an analog data receiver includes an analog todigital converter that is directly coupled to the analog output signalsfrom the capacitive touch screen device without intermediate analogstages. This circuit topology is in sharp contrast to the prior knownsolutions. The use of the novel features advantageously eliminates manypassive components which make new highly integrated circuit solutionsfeasible, as a single integrated circuit can now include the analog anddigital portions of a touch screen system with high manufacturing yieldand high reliability.

In FIG. 4, a block diagram of a conventional sigma delta analog todigital converter 400 is illustrated for the purposes of discussion. InFIG. 4 an analog input signal ‘u’ is input to a summing node 401 whichreceives the analog input signal u and subtracts the value of an analogfeedback signal supplied from a digital to analog converter labeled“DAC” coupled to the digital output. An integrator 405 receives thedifference from the summing node 401 and an analog to digital converter“ADC” converts the integrator 405 output to a digital value ‘v’corresponding to the analog input signal ‘u’. In a practical system theanalog to digital converter ADC may be implemented as a comparatorfollowed by a clocked sampling latch which quantizes the comparatoroutput into a series of serial data bits which, when averaged, form adigital representation corresponding to the analog input signal u. Thedigital output signal ‘v’ is also converted back to an analog voltage bythe digital to analog converter DAC for the negative feedback path tothe summing node 401.

In an aspect of the present application, it has now been surprisinglydiscovered that a novel analog data receiver for a touch screen circuitcan be formed from an analog to digital converter that is directlyconnected to the capacitive touch screen sense signal outputs, withoutthe use of the intermediate analog stages that are presently required inthe known prior approaches. Recognition of certain similarities betweenthe topology of a sigma delta analog to digital converter and thevarious analog stages of the prior known approach analog circuitryenables the creation of a novel circuit topology for an analog datareceiver. The novel analog data receiver uses only a sigma delta analogto digital converter that is directly connected to the analog outputs ofthe capacitive touch screen device.

FIG. 5 depicts in an aspect of the present application an illustrativeexample system 500. In system 500, an analog data receiver 510 isdirectly connected to an analog sense output signal SNS_IN from acapacitive touch panel 501 with a sense capacitor Csig. In FIG. 5, anintegrator 503 is formed by operational amplifier 507 with a biasvoltage coupled to a positive input terminal, and a negative inputterminal directly coupled to the analog input signal SNS_IN. A feedbackpath is formed by the feedback resistor RFB and the feedback capacitorCFB, coupled in parallel, and a summing node 504 is formed at thenegative input terminal of the operational amplifier 507. The output ofthe integrator circuit 503 is coupled to the positive input terminal ofa comparator circuit 505 formed by operational amplifier 509 and havinga voltage bias at the negative input terminal. The output of thecomparator 509 is coupled to a clocked sampling latch 508. The output ofclocked sampling latch 508 is a series of digital data bits output atthe node labeled SNS_OUT output that occur at a frequency that isdetermined by the sampling clock CLK. A buffer 511 and resistor R form adigital to analog converter in an output feedback path that is coupledto the summing node 504 at the negative input terminal of theoperational amplifier 507.

The circuit topology for the novel analog data receiver 510 can bedescribed mathematically as a transfer function:Vout=Vin*2πFstmCsigR  Equation 7

Where Fstm is the frequency of the input stimulus signal

The noise transfer function can be expressed as:Noise Vn1=Vp*2πFstmCpR  Equation 8

Therefore the signal to noise ratio (SNR) for the system of FIG. 5 canbe expressed as the ratio of output signal Vout to the noise Vn1 in thefrequency range of interest, or:

$\begin{matrix}{{SNR} = {\frac{Vout}{{Vn}\; 1} = \frac{{VinCsigFstm} \star \sqrt{3}}{{Cp} \star {\left( \sqrt{{Vp}^{2}} \right)\left( {{f\; 2^{3}} - {f\; 1^{3}}} \right)}}}} & {{Equation}\mspace{14mu} 9}\end{matrix}$

-   -   where the frequency f1 is less than the stimulus frequency Fstm,        and    -   frequency f2 is greater than the stimulus frequency Fstm.

As can be seen from Equation 7 above, the system gain can be simplycontrolled by increasing Vin, by increasing R, and by increasing thestimulus frequency Fstm, giving the circuit designer very flexibleoptions to tailor the performance and gain of the circuit to aparticular application.

By comparing the circuit topology of the illustrative example analogdata receiver 510 of FIG. 5 to the topology of the known prior solutioncircuit 300 of FIG. 3, it can be observed that the trans-impedanceamplifier 303 of the prior art circuit has a topology close to theintegrator circuit 503 of FIG. 5. The comparator circuit 505 withoperational amplifier 509 and the clocked sampling latch 508 has acircuit topology that is, in some ways, close to the circuit topology ofthe band pass filter 305 in the prior known solution of FIG. 3. Further,the capacitive touch screen device 501 has a band pass filter nature, sothat the touch screen itself can be said to provide pass filtering inthe analog signals. Thus by forming the analog data receiver 510 usingthe novel circuit topology shown in FIG. 5, the illustrative exampleincorporating aspects of the present application shown in FIG. 5provides a circuit topology with a two stage amplifier function that, incertain respects, forms a circuit topology near the topology of theprior known solutions, but very importantly the novel analog datareceiver 510 is formed while avoiding the use of the many passivecomponents of the prior known solution 310 shown in FIG. 3, and thenovel circuit topology of the present application advantageouslyintegrates the analog to digital converter function into the analog datareceiver, thereby surprisingly eliminating entirely many active andpassive components that are used in the prior known solutions.

In operation, the analog data receiver 510 in the system 500 receivesthe analog input data signal SNS_IN which corresponds to Vin*Csig. Thissignal corresponds to a change in a capacitance that results when atouch occurs on a touch sensitive panel 501 in FIG. 5. The detectedtouch can be, as shown in FIG. 2 above, a touch performed by a humanfinger. Alternatively a conductive stylus or dedicated conductivepointing device can perform the touch, such as are used in a signaturepanel touch screen display at a point of sale terminal.

The analog input signal SNS_IN is integrated by the integrator circuit503 and a corresponding integrated output voltage is output to thecomparator 505. This output voltage is coupled to the positive inputterminal of the operational amplifier 509, which compares the outputvoltage from the integrator 503 to a bias voltage Vbias at the negativeinput terminal. The output voltage of comparator 509 is then sampled bythe clocked sampling latch 508. In the illustrative and non-limitingexample arrangement shown in FIG. 5, the clocked sampling latch 508 isimplemented as an edge triggered data flip flop DFF. However, otherregister types can also be used, and if used in place of the edgetriggered data flip flop, these circuits form additional alternativearrangements that are also contemplated herein as aspects of the presentapplication. Two phase clocking schemes can be used as additionalalternative arrangements that are also contemplated herein.

The input signal CLK provides the sampling frequency for the edgetriggered clocked sampling latch 508. A new data bit will appear on theoutput of the latch 508 at each rising edge of the CLK signal. Theclocked sampling latch 508 thus performs a quantizing function, bysampling the analog output of the comparator 509 to produce quantizedsample data. As is known to those skilled in the art, the use of such aquantizer in analog to data converters allows for the use ofoversampling; that is, sampling the analog signal at a rate greater thanthe minimum Nyquist rate needed to reproduce the analog signal. Byoversampling, system error due to noise such as quantization noise inthe system may be removed or reduced from the digital output by lateraveraging the samples and by applying digital filtering techniques tothe digital data.

The output SNS_OUT from the clocked sampling latch 508 is an oversampledseries of quantized data bits. The filter and decimator 513 operates toform a digital representation of the analog input signal SNS_IN bydividing down the number of samples (performing decimation) and thesamples SNS_OUT can also be used in band pass filtering in the digitaldomain. Thus the digital data output at the terminal DO is serialdigital data that forms an accurate digital representation of the analoginput signal SNS_IN.

The feedback path formed by the buffer 511 and resistor R to the summingnode 504 forms a negative feedback path. Summer 504 subtracts an analogsignal corresponding to the current quantized output bit from the inputsignal, thus constantly correcting the output SNS_OUT to track changesin the input signal SNS_IN.

In a novel approach that is an additional aspect of the presentapplication which accrues certain advantages, the analog data receiver510 integrates the analog functions including and the analog band passfilter of the prior known solution along with the analog to digitalconverter to produce digital output data. This innovative approachunexpectedly and advantageously eliminates many passive componentsrequired by the prior solutions as well as two active operationalamplifiers used in the prior solutions. The resulting novel analog datareceiver thus requires far less silicon area, and is far more tolerantto process and temperature variations and variations in device size anddevice operation such as Vt variations, than the circuits used in theprior known approaches. The novel analog data receiver 510 is easilyintegrated to form a complete system on a chip (SOC) solution for atouch screen controller function, for example. Some example systemarrangements that form additional aspects of the present applicationcontemplated by the inventors and which fall within the scope of theappended claims are further described below.

FIG. 6 illustrates the signal to noise performance of an analog datareceiver circuit incorporating aspects of the present application bycomparing the SNR for the novel analog data receiver circuit, as shownin Equation 9, to the SNR obtained from the prior known solutionillustrated in FIG. 3 above. The signal to noise ratio for the priorknown solution circuit 310 can be determined for the first amplifierstage 303, the trans impedance amplifier, as:

$\begin{matrix}{{{SNR}\mspace{14mu}{prior}\mspace{14mu}{solution}} = {\frac{Vout}{Vnoise} = {{({VinCsig})/{Cp}} \star \sqrt{\left( {{Vp}\; 2} \right) \star \left( {{f\; 2} - {f\; 1}} \right)}}}} & {{Equation}\mspace{14mu} 10}\end{matrix}$

FIG. 6 depicts in a graph of SNR versus frequency overlying data plotsof the SNR obtained for each of the two circuits, the frequency plots ofthe SNR for novel aspects of the present application (labeled “new”) andfor the first stage 303 of the prior known approaches (labeled “old”).

As can be seen from examining the SNR graph depicted in FIG. 6, theoverall SNR function for the novel arrangements that include aspects ofthe present application such as analog data receiver circuit 510 isshown to have an SNR equivalent to the SNR for just the first stage 303,the trans-impedance amplifier, of the prior known solution 310. Becausethe second stage of the prior known solution, the Sallen Key band passfilter 305 in FIG. 3, will certainly add more noise to the overallsystem performance, circuits formed incorporating various aspects of thepresent application have a noise performance that is at least as goodas, and probably substantially better than, the circuits of the priorknown approach.

The SNR performance of the example arrangements described thus farconsidered the illustrative example of the analog data receiver circuit510 in FIG. 5. However this illustrative example analog data receiveruses a first order sigma delta ADC circuit. By extending the analog datareceiver of the illustrative arrangements to be a second order sigmadelta ADC circuit, additional improvement in the performance of thesystems incorporating aspects of the present application can beattained.

FIG. 7 depicts, for example, an alternative example arrangement of thenovel analog data receiver circuit in a second order circuit topology.In FIG. 7, an illustrative example system 700 includes a touch screen701 and an analog data receiver 710. The analog data receiver 710includes a second order sigma delta analog to digital converter. Theillustrative example data receiver 710 in FIG. 7 includes the componentsanalogous to the example analog data receiver circuit 510 in FIG. 5, andin addition, adds a second integrator circuit 723 between the firstintegrator circuit 703 and the comparator 709. As is known to those ofskill in the art, the use of the second order sigma delta ADC canimprove the ADC circuit performance.

FIGS. 8A and 8B present, in two graphs, the SNR performance obtained bythe example arrangements for the circuits described above. FIG. 8Adepicts the SNR obtained for the first order sigma delta ADC arrangementfor an analog data receiver as illustrated in FIG. 5. As can be seenfrom FIG. 8A, the SNR obtained is about 50 dB at a selected clockfrequency of 19.2 MHz. The SNR illustrated by the plot in FIG. 8B is forthe second order sigma delta ADC analog data receiver in theillustrative example arrangement of FIG. 7, which adds an additionalintegrator circuit 723 and a second buffer 711 to the first order sigmadelta ADC arrangement of FIG. 5. The SNR obtained is even better, asexpected, and it can be seen from examining the plot presented in FIG.8B that the SNR is now 62 dB at the example clock frequency of 19.2 MHz.

FIG. 9 depicts in another block diagram an illustrative example system900 and an example monolithic integrated analog data receiver circuit902. In system 900, a touch screen 901 has multiple analog sense outputscorresponding to rows of capacitors used in sense detection. Theillustrative example monolithic integrated circuit 902 includes aplurality of the analog data receiver circuits 910-1 to 910-N, such asare illustrated in FIGS. 5 and 7 above, each of these analog datareceiver circuits is coupled to one of the analog sense signals.

Because the novel arrangements that are aspects of the presentapplication reduce the silicon area needed for the integrated analogdata receiver, and because the novel circuits discovered by theinventors of the present application use fewer passive components foreach, the novel monolithic integrated circuit 901 can include severalsignal channels and provide a single analog data receiver chip for thetouch screen 901. Each of the illustrative example circuits 910-1 to910-N includes a sigma delta analog to digital converter including theintegrator circuits, shown as 903-1 to 903-N, coupled directly to analogoutput signals of the touch screen panel, and the comparator circuits905-1 to 905-N, and the decimators 913-1 to 913-N. The illustrativeexample of monolithic integrated circuit 902 has a digital data outputfor each of the analog sense signals from the touch screen 901.

In yet another alternative arrangement that incorporates certain novelfeatures of the present application, the high level of integration thatcan be advantageously achieved by use of the example integrated analogdata receiver circuits enables the analog data receiver function to beformed integral to the capacitive touch screen panel itself, eliminatingthe ribbon cable or flex cable connectors used in the prior knownsolutions, which further reduces costs and which further improvesperformance by removing resistive elements from the system.

FIG. 10 illustrates an example system 1000 that can advantageouslyincorporate various features of the present application. In FIG. 10, aprinted circuit board, module, or single monolithic integrated circuit1002 includes the analog data receiver 902 of FIG. 9 described above,for example, and also a digital processing function 1005 for processingthe data received from a touch screen display panel 1013. The digitalprocessing block 1005 can be implemented as a digital signal processoror DSP, mixed signal processor or MSP, a microprocessor, amicrocontroller, an advanced reduced instruction set machine (ARM), astate machine, and the like. The digital processing function 1005 iscoupled to a data output DATA by an interface block 1001. The datainterface may implement a serial bus protocol such as I2C, SDI, and thelike, for example. A clock and control block 1003 is provided to controltiming and provide control signals. A voltage stimulus signal providesthe sinusoidal excitation signals needed to drive the Vin signals to thesense capacitors disposed in the touch screen 1013. The touch screen1013 may be arranged with conductors in rows and columns, the columnsfor example receiving the sinusoidal excitation signals, and the rowsproviding the sense output signals. A capacitor is formed at theintersections of the rows and columns for detecting touch on the surfaceof the touch screen as shown in FIG. 2 above.

The capacitive touch screen device 1013 in FIG. 10 can be part of anydevice using a touch screen as an input. For example touch screen device1013 can be, without limitation, a smartphone, tablet computer, laptopcomputer, personal computer, desktop computer, point of sale terminal,music player, set top box, video player, internet browser, pager,consumer appliance, consumer control such as a thermostat or homesecurity system, and industrial controls.

The illustrative example arrangement circuit 1002 can be implemented assingle integrated circuit to form a system on a chip (SOC) touch screencontroller. This highly integrated device can perform the analog datareceiver functions as well as the digital signal processing required toprovide a complete solution for the use of the touch screen device 1013on a single integrated circuit. In an alternative illustrativearrangement of the present application, a pair of integrated circuitsmay be provided in a stacked die package to form an SOC or SIP solution.In a stacked die package, a first integrated circuit may include theanalog data receiver 902, while a second integrated circuit may includethe digital functions such as 1005, 1001, and 1003. Other arrangementsincluding integrated circuits having on-board RAM and on-boardnon-volatile memory are also contemplated as additional alternativearrangements that incorporate various features of the presentapplication and which accrue significant advantages by use of thevarious features disclosed herein.

In an example advantageous arrangement incorporating aspects of thepresent application that provides a space saving solution, theintegrated circuit 1002 is provided integrally with the touch screendevice 1013, that is, the packaged touch screen device 1013 includes thetouch screen controller 1002 without the need for ribbon connectors orflex cables between the devices as was conventionally done. Eliminationof these connectors reduces cost, saves space, and improves systemperformance by eliminating the resistance associated with the physicalconnectors of the prior known solution.

Although the example illustrative arrangements have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the present application as defined by the appended claims.

Moreover, the scope of the present application is not intended to belimited to the particular illustrative example arrangement of theprocess, machine, manufacture, and composition of matter means, methodsand steps described in this specification. As one of ordinary skill inthe art will readily appreciate from the disclosure, processes,machines, manufacture, compositions of matter, means, methods or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding example arrangements described herein may be utilizedaccording to the illustrative arrangements presented and alternativearrangements described, suggested or disclosed. Accordingly, theappended claims are intended to include within their scope suchprocesses, machines, manufacture, compositions of matter, means,methods, or steps.

What is claimed is:
 1. An analog data receiver circuit for a touchscreen device, comprising: a sigma delta analog to digital converterconfigured for direct connection to an analog output signal of a touchscreen device, and further comprising: an integrator circuit having aninput coupled for receiving the analog output signal from the touchscreen device and outputting an integrated output voltage; a comparatorcoupled to the integrated output voltage and a first bias voltage andoutputting a comparison voltage; a clocked sampling latch coupled to thecomparison voltage and to a clock signal and outputting quantized databits corresponding to samples of the comparison voltage; and a digitalfilter and decimator coupled to the output of the clocked sampling latchand outputting serial data bits which form a digital representationcorresponding to the analog output signal of the touch screen device. 2.The analog data receiver circuit of claim 1, wherein the integratorcircuit further comprises: an operational amplifier having a positiveinput terminal coupled to a bias voltage and a negative input terminalcoupled to the analog output signal and outputting the integratedvoltage output; a capacitor coupled in a feedback path from theintegrated voltage output to the negative input terminal; and a resistorcoupled in parallel and in the feedback path from the integrated voltageoutput to the negative input terminal.
 3. The analog data receivercircuit of claim 1, further comprising: a summing node at the input tothe integrator circuit and coupled to the analog output signal; and afeedback path coupling an inverted analog feedback voltage to thesumming node.
 4. The analog data receiver circuit of claim 3 and furthercomprising: a buffer in the feedback path coupled to receive the outputof the clocked sampling latch and outputting the analog feedback voltageto the summing node.
 5. The analog data receiver circuit of claim 4 andfurther comprising: a resistor coupled in the feedback path and disposedbetween the buffer and the summing node.
 6. The analog data receivercircuit of claim 5, wherein the buffer performs a digital to analogconversion and the analog feedback voltage has a level corresponding toa voltage represented by the serial data bits output by the clockedsampling latch.
 7. The analog data receiver circuit of claim 1, whereinthe comparator further comprises: an operational amplifier having apositive input terminal coupled to the output of the integrator circuit;and a negative input terminal of the operational amplifier coupled to abias voltage.
 8. The analog data receiver circuit of claim 1, whereinthe clocked sampling latch further comprises an edge triggered data flipflop.
 9. The analog data receiver of claim 1, wherein the decimatortakes samples of the quantized data bits output by the clocked samplinglatch and the decimator outputs a number of digital data bits lower thana number of quantized data bits.
 10. The analog data receiver of claim1, wherein the filter performs a digital band pass filtering of theserial data output by the clocked sampling latch.
 11. An integratedanalog data receiver circuit for a touch screen device, comprising: aplurality of inputs configured to receive analog sense outputs of atouch screen device; a plurality of analog data receiver circuits, eachof the plurality of analog data receiver circuits coupled to at leastone of the plurality of inputs, and each further comprising: a sigmadelta analog to digital converter further comprising: an integratorcircuit having an input directly receiving an analog sense output fromat least one of the plurality of inputs and outputting an integratedoutput voltage; a comparator coupled to the integrated output voltageand coupled to a first bias voltage and outputting a comparison voltage;a clocked sampling latch coupled to the comparison voltage and to aclock signal and outputting quantized data bits corresponding to samplesof the comparison voltage; and a digital filter and decimator coupled tothe output of the clocked sampling latch and outputting serial data bitsforming a digital representation corresponding to an analog sense outputsignal.
 12. The integrated analog data receiver circuit of claim 11,wherein within each of the sigma delta analog to digital converters, theintegrator circuit further comprises: an operational amplifier having apositive input terminal coupled to a bias voltage, and a negative inputterminal coupled to the analog sense output signal and outputting theintegrated voltage output; a capacitor coupled in a feedback path fromthe integrated voltage output to the negative input terminal; and aresistor coupled in parallel with the capacitor in the feedback pathfrom the integrated voltage output to the negative input terminal. 13.The integrated analog data receiver circuit of claim 11, wherein withineach of the sigma delta analog to digital converters, each of thecomparators further comprise: an operational amplifier having a positiveinput terminal coupled to the integrated voltage output of acorresponding integrator circuit; and a negative input terminal of theoperational amplifier coupled to a bias voltage.
 14. The integratedanalog data receiver circuit of claim 11, wherein within each of thesigma delta analog to digital converters, each of the decimators samplethe quantized data bits output by a corresponding clocked sampling latchand each decimator subsequently outputs a number of digital data bitsthat is lower than a number of quantized data bits sampled.
 15. Theintegrated analog data receiver circuit of claim 11, wherein within eachof the sigma delta analog to digital converters, the clocked samplinglatch further comprises an edge triggered data flip flop.
 16. Theintegrated analog data receiver circuit of claim 11, wherein each of thesigma delta analog to digital converters further comprises: a summingnode at the input to the integrator circuit; and a feedback pathcoupling an inverted analog feedback voltage to the summing node.
 17. Asystem for receiving user inputs from a touch screen device, comprising:a touch screen device configured to transmit analog output signalscorresponding to touches detected on a capacitive touch sensitivesurface of the touch screen device; a touch screen controller circuitthat is formed integral with the touch screen device, comprising: aplurality of analog data receiver circuits, each of the plurality ofanalog data receiver circuits coupled to at least one of the analogoutput signals and each further comprising: a sigma delta analog todigital converter further comprising: an integrator circuit having aninput coupled for receiving an analog signal directly from at least oneof the analog output signals and outputting an integrated outputvoltage; a comparator coupled to the integrated output voltage andcoupled to a first bias voltage and outputting a comparison voltage; aclocked sampling latch coupled to the comparison voltage and to a clocksignal and outputting quantized data bits corresponding to samples ofthe comparison voltage; a digital filter and decimator coupled to theoutput of the clocked sampling latch and outputting serial data bitsforming a digital representation corresponding to an analog signal atthe input; and a digital processing circuit coupled to receive theserial data bits as input data and configured to perform definedfunctions responsive to the touches detected on the touch screen device.18. The system of claim 17, wherein the touch screen controller circuitcomprises a monolithic integrated circuit including at least theplurality of analog data receiver circuits.
 19. The system of claim 18,wherein the touch screen controller circuit comprises a singlemonolithic integrated circuit including at least the plurality of analogdata receiver circuits and the digital processing circuit.
 20. Thesystem of claim 17, wherein the touch screen device further comprises adevice that is one selected from the group consisting essentially of asmartphone, a tablet computer, a music player, a video player, a laptopcomputer, a desktop computer, a consumer appliance, and an industrialcontrol.